This invention relates generally to the production of continuous linear material such as strands of glass, yarn or the like which is wound into packages on the collet of a winder. More specifically this invention relates to a speed control apparatus for controlling the rotational speed of the winder to maintain a constant strand tension.
A strand of heated thermoplastic material such as molten glass is normally packaged by winding the strand about the collet of a winder to form a collection of multiple layers. The multiple layers which accumulate about the collet increase the effective diameter of the collet thereby increasing the circumferential speed and tension upon the strand. The pulling force exerted by the winder effects the diameter of the strand. In order to maintain a constant strand tension and hence constant strand diameter throughout the package, the rotational speed of the winder can be decreased in proportion to the increasing package diameter to maintain a constant circumferential speed.
U.S. Pat. Nos. 3,771,324, 3,838,827 and 3,861,609, all assigned to the assignee of the present invention, disclose apparatus for processing linear elements such as molten glass. In each of these patents an analog control circuit using DC voltages and potentiometers was utilized to control the speed of the winder.
Speed controllers which control the speed of an electric motor in proportion to the frequency of an input signal are commercially available. It is known in the art to employ analog circuitry consisting of potentiometers, DC reference voltages, and circuitry for combining a plurality of voltages, such as summing amplifiers, to produce a variable DC voltage which is converted by a voltage controlled oscillator (VCO) to a variable frequency signal for controlling such speed controllers.
Although such motor control systems have proved generally successful, such analog systems inherently respond to a variety of parameters such as potentiometer wear, power supply voltage variations, and temperature changes. Any variation of the control DC voltage to the VCO due to such parameters will cause a resulting change in the output reference frequency thereby causing an undesirable change in motor speed. Such systems are often difficult to adjust because a plurality of potentiometers must be adjusted. Each adjustment is often dependent to some extent upon the other adjustments thereby making adjustment of the system tedious and time consuming. Therefore, it will be apparent that there exists a need for an apparatus for generating a controllable reference frequency which overcomes or reduces the above difficulties.